Because of the great interest in reducing the pollution of the atmosphere it becomes increasingly important that any nitrogen oxide gas which might result from chemical or refinery operation must be reduced to basic nitrogen before they are permitted to go into the atmosphere. There are a large number of nitrogen oxides, such as N.sub.2 O, NO, NO.sub.2, (NO).sub.2, N.sub.2 O.sub.3, (NO.sub.2).sub.2, N.sub.2 O.sub.5, etc., and referred to herein as "NOx." Several of these, such as NO.sub.2, (NO.sub.2).sub.2, and N.sub.2 O.sub.3 help form a large part of the brownish haze which is seen in smog in the larger cities. Most nitrogen oxide gases can support combustion at suitable temperatures. However, it is necessary to have a reducing atmosphere before the nitrogen oxides are reacted to free the nitrogen.
The prior art devices have a serious shortcoming that is overcome in the present invention, in that all of them rely on a burner operation with less than stoichiometric air for generation of combustibles to reduce the nitrogen oxides.
It is more convenient to operate the burner or heat source at stoichiometric air supply for the fuel which creates a hot gaseous atmosphere, and then cause the atmosphere to be reducing through separate addition to it of a hydrocarbon-steam mixture, or H2, CO, or hydrocarbon. The steam-hydrocarbon mixture is preferred because of typical reforming reaction as: EQU CH4+ H20 = CO+ 3H2
this reaction, which is endothermic, is caused to occur by heat available from the hot furnace atmosphere to cause the atmosphere to become reducing. It is also possible to cause the furnace atmosphere to become reducing through, again, separate introduction of an air-hydrocarbon mixture in which the hydrocarbon-air mixture contains less-than stoichiometric air for the hydrocarbon but enough air to prevent the presence of free carbon.
When the NOx are introduced into this region of reducing atmosphere the nitrogen is freed and the hydrogen and carbon monoxide are partially burned. With the nitrogen freed, the gases must be cooled below a temperature, above which, reoxidation of the nitrogen will occur. It is therefore necessary to move the gases from the first chamber into a second contiguous chamber where the temperature is dropped as rapidly as possible. This can be done by the introduction of cooling means, to reduce the temperature rapidly to below 2,000.degree. F. and preferably to about 1800.degree. F. This is well above the auto-ignition temperature for hydrogen and carbon monoxide so they will automatically ignite and be burned before the gases issue into the atmosphere, and it is well below the temperature at which substantial amounts nitrogen will automatically oxidize.